Palladium(II) NCS‐Pincer Complexes Mediated Regioselective Cross Dehydrogenative Alkenation of 2‐Arylthiophenes

In this report, we have synthesized two NCS pincer ligands by the Schiff base reaction of 3-((phenylthio)methoxy)benzaldehyde (P) with alkyl amines (tbutylamine (L1) and 1-adamantylamine (L2)). The palladium pincer complexes (tbutylamine = C1 and 1-adamantylamine = C2) of these ligands were synthesized by their reaction with PdCl2(CH3CN)2. The newly synthesized ligands and complexes were characterized using various techniques such as 1H, 13C{1H} Nuclear Magnetic Resonance (NMR), Ultraviolet–visible (UV-Visible), Fourier Transform Infrared (FTIR) Spectroscopy, and High-Resolution Mass Spectrometry (HRMS). The structure of ligand and its coordination mode with palladium precursor were studied with the help of single-crystal X-ray diffraction. The complexes showed distorted square planar geometry around the palladium center. The palladium pincer complexes were used as catalysts for the regioselective cross-dehydrogenative alkenation of 2-arylthiophene derivatives. The complex C2, where sterically bulky adamantyl ligand is part of the side arm showed a higher yield of alkenation reaction. Only 2.5 mol% catalyst loading was sufficient to achieve 74-95% yields of desired products with excellent functional group tolerance under mild reaction conditions. The poisoning experiments (PPh3 and Hg) showed the homogeneous nature of the catalytic process. The plausible mechanism of the reaction was proposed based on the control experiments and time-dependent HRMS studies.